The invention is directed to a bath for the electrolytic (galvanic) deposition of gold/tin alloy coatings at a pH below 3 consisting of (or consisting essentially of) 1 to 20 g/l gold in the form of alkali (e.g., sodium or potassium) and/or ammonium tetracyanoaurate (III), 0.1 to 10 g/l tin in the form of a water soluble tin salt, in a given case 0.005 to 1 g/l of an additional alloying metal in the form of a water soluble salt, an acid and a buffer or conducting salt.
The electrolytic codeposition of tin from gold baths is of interest for decorative and industrial uses. In the decorative industry, gold/tin alloy baths are employed for the production of gold colored coatings, above all white gold coatings, and in electrotechnology the good corrosion resistance and solderability of the tin in combination with gold offers advantages compared to other alloy coatings.
The common deposition of tin and gold is possible from alkaline and acid baths. In Kersten U.S. Pat. No. 1,905,105 there is described an alkaline electrolyte which contains potassium dicyanoaurate (I) and tin as potassium hydroxystannate (IV). However, gold/tin alloy coating are not deposited from baths of this type in glossy metallic form.
Tin in combination with gold can only be deposited from weakly acid baths at pH from 3.5 to 6 if the tin is divalent. The electrolyte thereby contains the gold in the form of potassium dicyanoaurate I, KAu(CN).sub.2. From German OS No. 1960047 and German AS No. 2523510 there are known such electrolytes. These baths are definitely not stable since the divalent tin is relatively easily oxidized to the tetravalent state, which under the stated conditions no longer will codeposit. Even protective materials such as complex formers, sulfonic acids, or soluble tin anodes cannot sufficiently prevent the oxidation.
There is known from German OS No. 2658003 and German Pat. No. 3012999 (and related Zilske U.S. Pat. No. 4,391,679), the codeposition of tin from strongly acid gold electrolytes at pH below 3, which contain the gold in the form of tetracyanoaurate (III) complexes, Au (CN).sup.-.sub.4. In these baths, the tin is deposited from the divalent and the tetravalent oxidation stage.
In German OS No. 2658003 there is described an electrolytic bath which contains 1 to 30 g/l gold as Gold (III)-cyanide complex, whereby to establish the pH and to stabilize the complex there are used hydrohalic acids which lead to considerable corrosion damage to the electrolytic plants. Besides during the electrolysis toxic halogen is developed which under the stated operating conditions escapes in gaseous form.